Dry Limit of Life

Mars on Earth?

A team of scientists from LSU, NASA, the Universidad Nacional Autonoma de Mexico and other research organizations has discovered an area of Earth that is shockingly similar to the surface of Mars.

Chris McKay.Credit: NASA Ames

This joint research effort has discovered clues from one of Earth’s driest deserts about the limits of life on this planet, and why past missions to Mars may have failed to detect life. The results of the group’s study were published this week in Science magazine, in an article titled "Mars-like Soils in the Atacama Desert, Chile, and the Dry Limit of Microbial Life."

NASA’s Viking missions to Mars in the 1970s showed the martian soil to be disappointingly lifeless and depleted in organic materials, the chemical precursors necessary for life. Last year, in the driest part of Chile’s Atacama Desert, the research team conducted microbe-hunting experiments similar to Viking’s, and no evidence of life was found. The scientists called the finding "highly unusual" in an environment exposed to the atmosphere.

"The Atacama is the only place on Earth (from which) I’ve taken soil samples to grow microorganisms back at the lab and nothing whatsoever grew," said Fred A. Rainey, an associate professor in biological sciences at LSU, who is a co-author of the study and an expert on microorganisms in extreme environments. "Normally, when you take a soil sample from any environment and you plate it on nutrient media, you see many different bacterial colonies growing there after a few days. But, in the case of the soils collected in some areas of the core region of the Atacama Desert, no or very few bacterial colonies appear, even after 20 days of incubation.

The Atacama desert is one of the driest places on Earth. Vivid colors belie the arid landscape of northern Chile where the Atacama Desert, one of the world’s driest, meets the foothills of the Andes. Here salt pans and gorges choked with mineral-streaked sediments give way to white-capped volcanoes. This scene was acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on NASA’s Terra satellite on October 28, 2001. Scientists are searching for traces of microorganisms in this extreme environment. Credit: USGS

"This is a very unique situation and, in some ways, represents an environment where the limits of microbial survival in arid environments have been reached or are close to being met."

Several LSU undergraduate students in biological sciences – Danielle Bagaley, Becky Hollen and Alanna Small – were also involved in the research and are co-authors of the article.

Chris McKay, who is based at NASA Ames Research Center in Moffett Field, Calif., is the expedition’s principal investigator.

"In the driest part of the Atacama, we found that, if Viking had landed there instead of on Mars and done exactly the same experiments, we would also have been shut out," McKay said. "The Atacama appears to be the only place on Earth Viking would have found nothing."

During field studies, the team analyzed Atacama’s depleted Mars-like soils and found organic materials at such low levels and released at such high temperatures that Viking would not have been able to detect them, said McKay. McKay also noted that the team discovered a non-biological oxidative substance that appears to have reacted with the organics – results that mimicked Viking’s results.

According to the researchers, the Atacama site they studied could serve as a valuable testbed for developing instruments and experiments that are better tailored to finding microbial life on Mars than the current generation.

Top, Death Valley, Calif.; bottom, Sojourner rover image of Mars

"It is considered by many that there is no microbial life on the surface of Mars and that the soils there are inhospitable to microbial life," said Rainey. "The soils in the core region of the Atacama Desert would seem to fall into this category and, thus, represent a great natural resource for the testing of life detection systems, as well as exploring what impact we would have on such environments on other planets."

McKay explained that he also sees the Atacama research as a tremendous opportunity for developing better technology.

"We think Atacama’s lifeless zone is a great resource to develop portable and self-contained instruments that are especially designed for taking and analyzing samples of the martian soil," McKay said.

More sophisticated instruments on future sample-return Mars missions are a necessity if scientists are to avoid contaminating future martian samples, McKay explained.

"We’re still doing the first steps of instrument development for Mars," McKay said, also noting that, recently, researchers have developed a method to extract DNA from soil without humans getting involved in processing the data.

The Viking 2 Lander.Credit: NASA

The reason Chile’s Atacama Desert is so dry and virtually sterile, the researchers say, is because it is blocked from moisture on both sides by the Andes mountains and by coastal mountains. At 3,000 feet, the Atacama is 15 million years old and 50 times more arid than California’s Death Valley.

The scientists studied the driest part of the Atacama, an area in what is called a "double rain shadow." During the past four years, the team’s sensor station has recorded only one rainfall, which shed a paltry 1/10 of an inch of moisture.

The team hypothesizes that it rains in the arid core of the Atacama on average of only once every 10 years.

Rainey said that he plans more trips with students to Chile to examine the fate of microorganisms in the air of the Atacama’s hostile and extreme environment.

The Atacama research was funded by NASA’s Astrobiology Science and Technology for Exploring Planets program, LSU, the National Science Foundation and several other organizations.The article was also authored by Rainey, Bagaley, Hollen and Small from LSU; Rafael Navarro-Gonzalez, Paola Molina and Jose de la Rosa from the Universidad Nacional Autonoma de Mexico; Richard Quinn of the SETI Institute in Mountain View, Calif.; Frank Grunthaner of the NASA Jet Propulsion Laboratory in Pasadena, Calif.; Luis Caceres of Instituto del Desierto y Departmento de Ingenieria, Quimica; and Benito Gomez-Silva of Instituto del Desierto y unidad de Bioquimica, Unversidad de Antofagasta in Antofagasta, Chile.